High speed shuttering is needed in photodetector arrays to eliminate image blur caused by smear and/or to isolate optical pulses that are closely spaced in time. Eliminating image smear is important in high frame-transfer rate applications where the image transfer time is comparable to or greater than the image stare or integration time, since image smear under such conditions can significantly corrupt image information. Exemplary systems that require short exposure times and/or are sensitive to smear include high-speed photography systems, target tracking systems, range gating systems, and real-time adaptive optic systems.
Many frame-transfer photodetector arrays of the CCD type are operated so that the image integration time, i.e., the image exposure time, is substantially longer than the image transfer time so that image smear normally does not become a problem. Such an approach, however, severely restricts the use of these types of imaging systems and prevents their use in applications where it is desired to provide a high frame-transfer rate or a short-exposure time. Other solid state image sensors that include a shutter function often can be arranged to provide high frame-transfer rates or short-exposure times, but in doing so they often lose the ability to achieve a high pixel fill factor or require a relatively high degree of complexity and, hence, give rise to increased costs and difficulty in fabrication.
It is desirable to provide a CCD imaging system using a high-speed electronic shutter that can be specifically incorporated into the structure of the CCD, e.g., of a back-illuminated, frame-transfer CCD, to provide a device having a simpler structure that can be produced at reasonable costs. Such a device should be arranged to operate in a manner such that smear is substantially reduced, or eliminated, a high pixel fill factor (substantially at, or close to, 100%) is achieved, flexible integration times are made available, low-noise operation occurs, and near-reflection-limited quantum efficiencies over the visible spectrum can be simultaneously obtained. While other solutions to the problem of image smear, when the integration time is comparable to or less than the transfer time, have been proposed, none has been able to achieve such desired overall operation. For example, it has been suggested that image smear can be removed by performing post-processor data operations on the image data using suitably designed algorithms. Such an approach, however, is accomplished at the expense of time and increased hardware cost. Another proposed approach is to place an electro-optic shutter, such as a Pockel or Kerr cell, in front of the CCD. While such electro-optic shutters can have relatively fast switching times, e.g., on the order of nanoseconds, they suffer from other problems, such as requiring very high voltages for operations (i.e., often in the kilovolts range). They also tend to produce a relatively high (e.g. a 50%) loss of unpolarized light, are temperature sensitive, and tend to generate optic aberrations.
In still other approaches, electronic shutters have been proposed to be fabricated into a CCD structure by various methods. A typical example is an interline-transfer CCD wherein photoelectrons collected in photosensitive regions are transferred into the channel of an adjacent CCD at the end of an image capture operation. The adjacent CCD channel is covered by a blocking layer which prevents further collection of photoelectrons. In such a system, the image signal can be clocked out of the chip with reasonably negligible image smear. Shuttering by this technique requires only the time needed to transfer the charge from the photosite to the CCD channel (typically a single clock cycle). However, in such structures the pixel fill-factor is considerably reduced (i.e., well below 100%) because a significant portion of each pixel region has to be used by the CCD transfer channel.
An example of another approach is a frame-transfer CCD imaging system which shutters by transferring the entire image, i.e., all of the pixels thereof, from the imaging array into a frame-store array wherein the frame-store array is covered by an opaque material. Shuttering in this manner, however, creates a position-dependent-pixel image smear with the smear effect occurring and being most prominent for the pixels in the imaging array thereof that are positioned furthest from the frame-store array. The number of "smear" photoelectrons in such pixels is inversely proportional to the clock rate multiplied by the number of pixels in an imaging array column.
Still other CCD imagers create a shutter function by placing a photosensitive region over the CCD transfer channels. Such a method can provide pixel photosites with a high pixel fill-factor and can shutter the photoelectron signal relatively quickly. Unfortunately, however, the photosensitive layer causes other operating problems for the CCD, such as image lag, and the system has high operating voltage requirements and tends to produce relatively large and undesirable dark currents.